Temperature sensitive devices can be found in virtually any field of technology. Typically a temperature sensitive device is designed to operate within a certain pre-defined temperature range. As long as the operational temperature remains within this pre-defined temperature window the device is in an allowed operational state. If for some reason the device is overdissipated, the operational temperature of the device can increase to a level above the allowed maximum temperature so that the performance of the device is degraded and/or the device is damaged. To prevent such a temperature sensitive device from entering into an unallowed operational temperature state a large number of circuits and methods have been devised in the prior art.
One prior art approach is to sense the operational temperature of the device and compare it to a pre-defined maximum temperature. If the operational temperature of the device surpasses the pre-defined maximum temperature, the power supply of the device is interrupted in order to stop the device from operating. After a pre-defined amount of time the device is reconnected to the power in order to restart operating the device after the cooling period.
This approach has the disadvantage that it is not adaptive to the particular circumstances of the overheating. For example the overheating of the device can be due to an external short circuit. If the short circuit is not removed consecutive attempts will be made to reconnect the device to the power supply whereby each time the amount of energy which can safely be dissipated by the device decreases. This can lead to destruction of the device. Also if the overheating is only of a transient character--for example due to a transient overload--the amount of time required for cooling down the device can be smaller than in other circumstances.
This phenomenon is particularly relevant to power switches. Switched mode power supplies usually use a high-voltage, high-current MOSFET to switch current on and off in the primary winding of an output transformer. Typically the MOSFET is controlled by a pulse width modulation circuit. In case of a short circuit or overload on the secondary side of the transformer such a power supply requires some protection against over heating due to excessive currents. Known devices for application in such an environment include the controllers MC33364 and MC33368 which are commercially available from MOTOROLA, INC. These controllers feature some kind of over temperature protection and shut down capability.